The thermal and structural performance of geopolymer-coated polyurethane foam–phase change material capsules/geopolymer concrete composites was investigated. Three groups of concrete composites were prepared. The first was pure geopolymer (GP, control sample), the second was a GP/polyurethane foam (F) concrete composite, and the third was GP-coated polyurethane foam-phase change material capsules (GP-F-PCM)/GP concrete composites. Three different percentages of foam and GP-F-PCM capsules (25%, 50%, and 75%) were used in the composites. Thermal and U-value tests were conducted for all composites to characterize their peak temperature damping and insulation performances. The addition of 75% foam has been noticed to increase the back-surface temperature by 5.9 °C compared to the control sample. This may be attributed to the degradation of foam into low molecular constituents in the presence of a strong alkali. However, a temperature drop of 12.5 °C was achieved by incorporating 75% of GP-F-PCM capsules. The addition of 50% foam as a sandwich layer between two halves of a geopolymer concrete cube is also investigated. It was found that inserting a foam layer reduced the back-surface temperature by 3.3 °C, which is still less than the reduction in the case of GP-F-PCM capsules. The compressive strength was tested to check the integrity of the prepared concrete. At 28 days of aging, the compressive strength dropped from 65.2 MPa to 9.9 MPa with the addition of 75% GP-F-PCM capsules, which is still acceptable for certain building elements (e.g., nonloadbearing exterior walls). Generally, the best results were for the GP-F-PCM composite capsules as a heat insulator.
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